The present invention relates generally to implantable, radially expandable medical prostheses which are frequently referred to as stents.
Stents are well known and commercially available. They are, for example, disclosed generally in U.S. Pat No. 4,655,771, Wallsten; U.S. Pat. No. 5,061,275, Wallsten et al., and U.S. Pat. No. 5,645,559, Hachtmann et al. Stent devices are used within body vessels of humans for a variety of medical applications. Examples include intravascular stents for treating stenoses, stents for maintaining openings in the urinary, biliary, tracheobronchial, esophageal, and renal tracts, and vena cava filters.
A delivery device is used to deliver the stent in a compressed state to a treatment site through vessels in the body. The flexible nature and reduced radius of the compressed stent enables it to be delivered through relatively small and curved vessels. In percutaneous transluminal angioplasty, an implantable endoprosthesis is introduced through a small percutaneous puncture site, airway, or port and is passed through various body vessels to the treatment site. After the stent is positioned at the treatment site, it is deployed by expansion to contact the vessel wall and the delivery device is removed. The stent remains in the vessel at the treatment site as an implant.
Stents must exhibit a relatively high degree of biocompatibility since they are implanted in the body.
Stents are typically made of metal, but have also been made using polymer materials of varied composition and in varied conformations obtained by a variety of processing techniques.
U.S. Pat. No. 5,500,013, Buscemi , et al., describes a biodegradable drug delivery vascular stent.
U.S. Pat. No. 5,527,337, Stack, et al., describes a bioabsorbable stent and method of making the same.
U.S. Pat. No. 5,591,222, Susawa, et al.,describes a device for dilating ducts in vivo, comprising a balloon-tipped catheter to which a cylindrical stent prepared by knitting or braiding or weaving biodegradable fibers to easily reduce diameter of the cylinder to a predetermined value is attached in a compressed condition, a method for preparing the device and a stent.
U.S. Pat. No. 5,591,224, Schwartz, et al., describes a bioelastomeric intraluminal stent comprising fibrin and elastin capable of providing a treatment of restenosis.
U.S. Pat. No. 6,245,103, Stinson, describes a bioabsorbable self-expanding stent formed from helically wound and braided filaments of bioabsorbable polymers such as PLA, PLLA, PDLA, and PGA.
CA 2025626, describes a biodegradable infusion stent used to treat ureteral obstructions. The application describes an extruded material construction made of epsilon-caprolactone; glycoside and L(−) lactide. The document describes a method for incorporating radiopaque materials such as barium sulfate into the polymer.
U.S. Pat. No. 4,950,258, Kawai et al., describes a biodegradable molded product having a first shape. The molded product is deformed at an elevated deforming temperature to form a second shape. The product is then cooled. When the product is reheated to a prescribed temperature, the product recovers the first shape.
U.S. Pat. No. 5,032,679, Brandley et al., describes a glycosaminoglycoside (GAG) composition made of tetrasaccharide units derived from heparin/heparin sulfate. The composition has use in preventing proliferation of smooth muscle cells.
U.S. Pat. No. 5,061,281, Mares et al., describes a medical device made from a resorbable homopolymer derived from the polymerization of an α-hydroxy carboxylic acid. The resorbable homopolymer has an average molecular weight of from 234,000 to 320,000 as measured by gel permeation chromatography.
Balloon expandable polymer stents typically have high radial elastic recoil when the balloon pressure is released, which can cause the stent to retract away from the vessel wall and migrate. This is particularly a problem with biodegradeable polymer stents. One design approach for compensating for low material properties is to increase the thickness of the structural element. The drawback of this approach is that it conflicts with another common design goal of angioplasty devices which is to minimize profile so as to make as small a puncture for the introduction site in the patient as possible.
Therefore there is a need for a method of minimizing elastic recoil in expanded polymer stents, especially with biodegradeable polymer stents.